Variable optical attenuators are becoming increasingly important in dense wavelength-division multiplexing (DWDM) optical fiber transmission systems. Variable attenuators are used to vary the amount of loss light experiences as it passes through the device, ranging from low loss (&lt;1 dB) to very high loss (&gt;30 dB). The loss mechanism for variable attenuators can be coupling loss between fibers, polarization loss, absorption loss, scattering loss, or any combination of these.
A variable attenuator based on coupling loss is typically composed of two fibers whose separation is controlled via mechanical motion. As the separation between fibers increases, the amount of loss also increases (see for example Benner et al., "Low-reflectivity in-line variable attenuator utilizing optical fiber tapers" J. Lightwave Technol., Vol 18, p 7, 1990).